1. Field of the Invention
The present invention relates to a microprocessor and a one-chip microcomputer, especially to an input-output port thereof.
2. Description of the Prior Art
A microcomputer can be connected to external memories such as a read only memory and a random access memory. A one-chip microcomputer has a memory expansion mode where an external memory is used such as a static random access memory or an electrically erasable read only memory. It has an input-output port including pins for address signals and for data signals to be connected to an external address bus and to an external data bus, so that address signals and data signals are transmitted through the external buses. A memory has pins for address signals and pins for data signals separately. Then, the pins for address signals and for data signals of the microprocessor or the microcomputer are connected through the external address bus and the external data bus to the counterparts of the memory, respectively. However, the number of pins assigned for address and data signals of a one-chip microcomputer is large, and this limits the number of input-output ports which can be included in the one-chip microcomputer.
It is desirable that a one-chip computer has input-output ports as many as possible. Then, in order to reduce the number of address and data pins, multiplex pins are provided to transmit address and data signals in time division and they are connected to a multiplex bus. Then, the number of the pins needed for address and data signals is reduced, while the number of input-output ports can be increased. On the other hand, a memory has pins for address signals and for data signals separately. Therefore, an external circuit is needed to be connected between the multiplex bus and the memory in order to separate address and data signals, and this increases a cost of the system. This is a disadvantage of such multiplex bus.
A peripheral device such as a gate array or an application specific standard product (ASSP) often adopts multiplex pins for address and data signals in order to increase the number of input-output ports. In such a case, an external circuit is needed to connect address and data buses from a microcomputer to the multiplex bus, and this increases a cost of a computer system.